1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission configured by connecting a hydraulic pump and a hydraulic motor via a hydraulic closed circuit and configured to enable variably controlling of the capacity of at least either of the hydraulic pump or the hydraulic motor, shifting the input revolution speed of the hydraulic pump and acquiring the output revolution speed of the hydraulic motor.
2. Description of Background Art
Various configurations of a hydrostatic continuously variable transmission are known. For example, hydrostatic continuously variable transmissions have been proposed and disclosed in JP-A No. H6-42446; JP Patent No. 2920772; JP-A No. H9-100909 and JP-A No. 2005-256979 by the applicants. These hydrostatic continuously variable transmissions disclosed in these patents and applications are each provided with a swash plate type plunger pump, a swash plate type plunger motor and a hydraulic closed circuit that connects a discharge port and a suction port of the swash plate type plunger pump to a suction port and a discharge port of the swash plate type plunger motor. The hydrostatic continuously variable transmission is configured so that a pump swash plate is driven by an engine, a pump cylinder and a motor cylinder are connected and are arranged on an output shaft, the rotation of a motor swash plate is regulated, and an angle of the motor swash plate can be variably adjusted.
The hydrostatic continuously variable transmissions disclosed in JP-A No. H6-42446; JP Patent No. 2920772; JP-A No. H9-100909 and JP-A No. 2005-256979 are not a genuine hydrostatic continuously variable transmission (HST) and are a hydromechanical transmission (HMT) in which mechanical power transmission is also combined. A lockup mechanism is provided that closes the hydraulic closed circuit to make the lockup executed when the quantity of oil that flows from a hydraulic pump into a hydraulic motor decreases according to the variable control of a swash plate inclined angle. The input rotation of the hydraulic pump and the output rotation of the hydraulic motor are substantially synchronous in a power transmission process in which the hydraulic motor is rotated by oil discharged from the hydraulic pump.
As described above, in a lock-up operation, the hydraulic closed circuit that connects the hydraulic pump and the hydraulic motor is closed and a flow of oil between the pump and the motor is cut off. The lock-up operation is required to be executed only when the quantity of oil that flows from the hydraulic pump into the hydraulic motor decreases and the input rotation of the hydraulic pump and the output rotation of the hydraulic motor are substantially synchronous. However, the former lock-up mechanisms disclosed-in the JP-A No. H6-42446; JP Patent No. 2920772; JP-A No. H9-100909 and JP-A No. 2005-256979 are configured so that the hydraulic closed circuit can be closed even when the input rotation of the hydraulic pump and the output rotation of the hydraulic motor are not substantially synchronous and lockup may be executed beyond a range of appropriate ratio (gear ratio) by some control and others.